The present invention relates to a method for rapidly introducing genes into germplasm which involves the use of crossing, backcrossing, intense selection and agronomic trait selection. More specifically, the method comprises the steps of: (a) crossing two parents of two different species, one of the parents (the donor parent) contains the gene of interest; (b) backcrossing the resultant progeny with the parent not containing the gene of interest (recurrent parent) for two to four backcrosses; (c) performing intense selection on a segregating generation of the progeny from the backcrossing step; and (d) selecting final germplasm on the basis of desired agronomic traits. The present invention further relates to two new and distinctive high yielding Pima Bt (Bacillus thuringiensis) cotton cultivars, designated 926 B Pima and 930 B Pima, which have been prepared in accordance with the method of the present invention.
The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference, and for convenience are referenced in the following text by author and date and are listed alphabetically by author in the appended bibliography.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm.
Pureline cultivars, such as generally used in cotton and many other crops, are commonly bred by hybridization of two or more parents followed by selection. The complexity of inheritance, the breeding objectives and the available resources influence the breeding method. Pedigree breeding, recurrent selection breeding and backcross breeding are breeding methods commonly used in self pollinated crops such as cotton. These methods refer to the manner in which breeding pools or populations are made in order to combine desirable traits from two or more cultivars or various broad-based sources. The procedures commonly used for selection of desirable individuals or populations of individuals are called mass selection, plant-to-row selection and single seed descent or modified single seed descent. One, or a combination of these selection methods, can be used in the development of a cultivar from a breeding population. Descriptions of breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Allard, 1960; Welsh, 1981; Fehr, 1987; Mayo, 1987).
Pedigree breeding is primarily used to combine favorable genes into a totally new cultivar that is different in many traits than either parent used in the original cross. It is commonly used for the improvement of self-pollinating crops. Two parents which possess favorable, complementary traits are crossed to produce an F.sub.1 (filial generation 1). An F.sub.2 population is produced by natural selfing of plants or by physically selfing the plants. Selection of desirable individual plants may begin as early as the F.sub.2 generation wherein maximum gene segregation occurs. Individual plant selection can occur for one or more generations. Successively, seed from each selected plant can be planted in individual, identified rows or hills, known as progeny rows or progeny hills, to evaluate the line and to increase the seed quantity, or, to further select individual plants. Once a progeny row or progeny hill is selected as having desirable traits it becomes what is known as a breeding line that is specifically identifiable from other breeding lines that were derived from the same original population. At an advanced generation (i.e., F.sub.5 or higher) seed of individual lines are evaluated in replicated testing. At an advanced stage the best lines or a mixture of phenotypically similar lines from the same original cross are tested for potential release as new cultivars.
The single seed descent procedure in the strict sense refers to planting a segregating population, harvesting one seed from every plant, and combining these seeds into a bulk which is planted the next generation. When the population has been advanced to the desired level of inbreeding, the plants from which lines are derived will each trace to different F.sub.2 individuals. Primary advantages of the seed descent procedures are to delay selection until a high level of homozygosity (e.g., lack of gene segregation) is achieved in individual plants, and to move through these early generations quickly, usually through using winter nurseries.
The modified single seed descent procedures involve harvesting multiple seed (i.e., a single lock or a simple boll) from each plant in a population and combining them to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. This procedure has been used to save labor at harvest and to maintain adequate seed quantities of the population.
Backcrossing is the preferred method of transferring one or more simply inherited genes from one variety (the donor parent) to a second variety (the recurrent parent). The F.sub.1 is continually crossed with the recurrent parent until the recurrent parent is recovered. Backcrossing is simple, requires small plant populations and generates results quickly. The accumulation of genes from the recurrent parent increases at a rate of 50% with each backcross.
The number of backcrosses required to recover the recurrent parent increases as the genetic distance between the donor and recurrent parent diverge. Backcrosses involving interspecific parents represent extreme genetic distance. Deleterious genetic interactions and even problems at the chromosome level are common. Numerous backcrosses are necessary to recover the recurrent parent.
The recovery of the recurrent parent is based on the probability of moving closer to that parent by 50% with each backcross, randomly. When deleterious genetic interactions occur, even with 99.99% of the genome of the recurrent parent reclaimed, it may not be sufficient. A gene or many genes responsible for the deleterious genetic interactions may be still present in the 0.01% of the donor parent DNA. Insignificant ratios of recurrent parent/donor parent DNA can have significant consequences in plant genomes.
Selection for desirable traits can occur at any segregating generation (F.sub.2 and above). Selection pressure is exerted on a population by growing the population in an environment where the desired trait is maximally expressed and the individuals or lines possessing the trait can be identified. For instance, selection can occur for disease resistance when the plants or lines are grown in natural or artificially-induced disease environments, and the breeder selects only those individuals having little or no disease and are thus assumed to be resistant.
Promising advanced breeding lines are thoroughly tested and compared to popular cultivars in environments representative of the commercial target area(s) for three or more years. The best lines having superiority over the popular cultivars are candidates to become new commercial cultivars. Those lines still deficient in a few traits are discarded or utilized as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from seven to twelve years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task is the identification of individuals that are genetically superior because, for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental lines and widely grown standard cultivars. For many traits a single observation is inconclusive, and replicated observations over time and space are required to provide a good estimate of a line's genetic worth.
The two cotton species commercially grown in the United States are Gossypium hirsutum, commonly known as short staple or upland cotton and Gossypium barbadense, commonly known as extra long staple (ELS) or, in the United States, as Pima cotton. Upland cotton fiber is used in a wide array of coarser spin count products. Pima cotton is used in finer spin count yarns (50-80) which are primarily used in more expensive garments. Other properties of Pima cotton are critical because of fiber end use.
Cotton is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop stable, high yielding cotton cultivars that are agronomically sound. The reasons for this goal are obviously to maximize the amount and quality of the fiber produced on the land used and to supply fiber, oil and food for animals and humans. To accomplish this goal, the cotton breeder must select and develop plants that have the traits that result in superior cultivars.
G. barbadense is a cultivated species of cotton denoted by the genome (AD)2. Peru is the proximate area considered to be the center of origin for the species. G. barbadense varieties grown in the United States are commonly referred to as "American Pima", Pima or ELS cottons. The terms "American Pima", Pima and ELS are elucidated in the Definitions listed below.
It is often desired to introduce traits from one cotton species into the other. Such interspecific crosses are generally hard to work with because the chromosomes of the distinct species involved do not pair well in the progeny. The consequence is a large number of progeny that are very poor relative to either parent post F.sub.1 generation. Interspecific crosses between upland and Pima species, where selection toward the Pima parent is desired, is further complicated by the fact that Pima fiber traits are held to very high standards.
The goal of a commercial cotton breeding program is to develop new, unique and superior cotton cultivars. In cotton, the important traits include higher fiber (lint) yield, earlier maturity, improved fiber quality, resistance to diseases and insects, tolerance to drought and heat, and improved agronomic traits. The breeder initially selects and crosses two or more parental lines, followed by generation advancement and selection, thus producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via this procedure. The breeder has no direct control over which genetic combinations will arise in the limited population size which is grown. Therefore, two breeders will never develop the same line having the same traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions, and further selections are then made, during and at the end of the growing season. The lines which are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments, with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce, with any reasonable likelihood, the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of research moneys to develop superior new cotton cultivars.
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, and the grower, processor and consumer; for special advertising and marketing and commercial production practices, and new product utilization. The testing preceding the release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
The development of new cotton cultivars requires the evaluation and selection of parents and the crossing of these parents. The lack of predictable success of a given cross requires that a breeder, in any given year, make several crosses with the same or different breeding objectives.
The cotton flower is monecious in that the male and female structures are in the same flower. The crossed or hybrid seed is produced by manual crosses between selected parents. Floral buds of the parent that is to be the female are emasculated prior to the opening of the flower by manual removal of the male anthers. At flowering, the pollen from flowers of the parent plants designated as male, are manually placed on the stigma of the previous emasculated flower. Seed developed from the cross is known as first generation (F.sub.1) hybrid seed. Planting of this seed produces F.sub.1 hybrid plants of which half their genetic component is from the female parent and half from the male parent. Segregation of genes begins at meiosis thus producing second generation (F.sub.2) seed. Assuming multiple genetic differences between the original parents, each F.sub.2 seed has a unique combination of genes.
There is a need in the art to increase the efficiency of breeding new cultivars having agronomically improved traits, especially new cultivars derived from interspecies. There is also a need in the art to develop new cultivars of Pima cotton which contain the Bt gene. The present invention describes a method for rapidly introducing genes into germplasm, such as the Bt gene (Bollgard.RTM.) into Pima cotton, and thus satisfies these needs.